Slab steepening and rapid mantle wedge replacement during back-arc rifting in the New Hebrides

The effects of the composition and angle of the subducting slab and mantle wedge flow on tectonic and magmatic processes in island arcs and associated back-arcs are poorly understood. Here we analyse the ages and compositions of submarine lavas from the flanks and the floor of the back-arc Futuna Trough some 50 km east of Tanna Island in the New Hebrides arc front. Whereas >2.5 Ma-old back-arc lavas formed from an enriched mantle source strongly metasomatized by a slab component, the younger lavas show less slab input into a depleted mantle wedge. The input of the slab component decreased over the past 2.5 million years while the enriched mantle was replaced by depleted peridotite. The change of Futuna Trough lava compositions indicates rapid (10 s of km/million years) replacement of the mantle wedge by corner flow and slab steepening due to rollback, causing extensional stress and back-arc rifting in the past 2.5 million years.

region, especially based on previous geodynamic modeling studies.Both poloidal flow and toroidal flow are intensive in this region (Schellart et al., 2007).Perhaps the authors can provide a 3D cartoon to show a more realis.ctectonic and magma.cmode.Some minor comments: • Line 64, Fig. 1: The figure need to be polished.The authors are suggested to use the same scale color bar in Fig. 1a and b.The loca.on of Fig. 1b should be marked in Fig. 1a.
• There are many loca.onnames (a bit confusing).The authors know the area very well, but I do have a hard .me to follow the names.Please label or draw the boundaries of the geological units as well as the geographic units in the figure explicitly (e.g., Futuna backarc, Futuna flank, Tanna volcanoes, Tana arc front).What is Futuna back-arc (e.g., Line 196)?The Futuna island?Where is Tanna arc front?
• Lines 83-102: It's good to discuss the onset of the ri2ing, although it's debated.Could the authors provide the preferred onset .me of the ri2ing (and the reasons) based on the previous studies?
• Figure 2: "Tanna young" samples have higher K2O contents than that of the "Tanna old".

What does this indicate?
• Line 185: It's good to mark the absolute age by linear color bar in Fig. 2. Same for the Figs.
• Line 188, Fig. 3: Yellow line is not for visualiza.on in the plot.
• Line 229: I agree with the "6.5 Ma", but doubt about "2.6".What is the evidence that the pre-ri2 magma.cac.vity ended 2.6 Ma ago • Line 257: The whole sec.on is very interes.ng.
I have enjoyed reading the manuscript, the authors used details geochemical, geochronological (Ar-Ar), and isotopic (Sr-Nd-Pb) data of basaltic rocks from Futuna trough/island to identify an interesting geochemical compositional change in the lava that formed before and after 2.5 Ma, that gives an indication about the change in mantle source from enriched to depleted ones and related these observations to slab rollback and opening of back-arc basin.After reading this MS, I can say that the authors did a very good job making the MS ready for publication after a few more suggestions.
There are some major comments for the authors to be considered: 1-Can you add a section about the petrographical description of the samples (+ images) in the supplementary section?L182: one sentence is needed to summarize this new age data.L198: This is not clear, there is no difference between Nd values between old and young Futuna Flank data?And the same for Ce/Pb ratio and even the Tanna samples?? L272: Some other ratios Ba/Nb, U/Nb, Pb/Nd, can help in your discussion and show the differences in slab contributions between the two types, please see Gamaleldien et al, 2020, SR (start of subduction paper).I like to see these ratios vs. Pb, Nd, Sr isotopes.These figures can be added to supplementary.This is an interesting, valuable paper.The different lines of evidence the authors use to build their story -rock ages, geochemistry, location, tectonic history -are effectively described and synthesized.The tectonic setting, strategy and conclusions are unique, so the paper is definitely appropriate for Nature Communications.
Nonetheless, the paper can be improved for greater clarity and impact.
Clarity -To be fully understood and effective, the paper needs some work.In a short work, readers will compare text and figures as they read, so the the figures should to be explicit and consistent.
1) The most important item is Fig. 1.The lettering in 1a is tiny (to my eyes).Every feature mentioned in the Results and especially Discussion (Evolution of the tectonic and magmatic processes) should be labelled so that readers can correlate the upper plate locations with interpretations of the sample chemistry (Geochemical evolution of the magmas) and processes occurring at depth (Implications of the geochemical evolution for tectonic processes).A box showing the location of 1b on Fig. 1a would be useful.
2) Also, in general, the figure captions are brief and should be expanded to cover everything on the diagram.Some examples -no explanation the different symbols on Fig. 1b, no explanation of the yellow field on Fig. 4, samples identified only as numbers on Fig. 3 legend (these are in the caption but are better on both), explanation of open green circles on Fig. 6.
3) Supplementary data: Some small editing needed -Supplementary Data 1 has all geochemical data for samples (Table S1) and standards (Standards Tab).Supplementary Data 2 has Ar dates and analytical parameters in Tabelle 1 and nothing in Tabelle 2.

Broadening the impact:
The authors emphasize that the mantle flow rate they infer is fast (line 55), so some comparisons with rates of mantle flow inferred for other arcs is needed.Confirmation of results with those of with Heuret and Lallemand is good, but comparison with mantle wedge flow rates estimated in the literature elsewhere would heighten the impact of the paper and enhance the value of the authors' conclusions.
The reviewer comments are in normal and our replies in bold text.

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): This MS is very interesting investigating the early rifting process and the associated mantle flow pattern during back arc basin spreading by geochemical data.The logic of the story is quite clear (although the geodynamic evolution needs further demonstration), and the supporting evidence is robust.My overall suggesting is moderate revision.Below are some comments/suggestions that may help improving the MS.
1.I wonder if there is any other evidence supporting the slab steepening process.In the Izu-Bonin-Marina subduction zone, slab steepening was suggested by the gradual narrowing area of the arc magmatism (Ishizuka et al., 2009) and the migration of the rear-arc volcanism towards the trench (Miyazaki et al., 2020).The more slab melt component found in the rear-arc basalts of the Izu-Bonin arc also reveals a deep mantle condition at which the temperature is high enough to induce slab melt (Miyazaki et al., 2020).I think it is necessary to find more evidence supporting your argument that the New Hebrides 2. Slab-deepening process in the Izu-Bonin subduction zone leads to a series of rear-arc volcanisms featured by increasingly enriched features with time (Miyazaki et al., 2020).This is contrary to the arc geochemistry variation shown in this study which is characterized by the increase in depletion.I think a comparison could be made between the two subduction zones to find the key process controlling the differences, which may better explain your ideas.We believe that the steep slab allows rapid inflow of unmetasomatized mantle which may not be the case in the Izu-Bonin.MORB-like mantle is also observed in back-arcs of other subduction zones with steep slabs like Tonga, South Sandwich and the Marianas and we have mentioned this in the discussion.4. The New Hebrides subduction evolved for quite a while (>10 Myr), and the subducted slab is quite deep, reaching the mantle transition zone.Why slab steepening occurred at this particular moment (i.e., 2.5 Ma)?We believe that the slab rollback has probably occurred in several phases with slab steepening and the event at 2.5 Ma was only the last in a series o such events but this is of course highly speculative.
5. Further discussion is needed on the mantle flow pattern and the depleted mantle source region, especially based on previous geodynamic modeling studies.Both poloidal flow and toroidal flow are intensive in this region (Schellart et al., 2007).Perhaps the authors can provide a 3D cartoon to show a more realistic tectonic and magmatic mode.• Figure 2: "Tanna young" samples have higher K2O contents than that of the "Tanna old".

What does this indicate?
We have checked and the old and young Tanna lavas are similar in K2O contents at a given SiO2.The relatively constant compositions are also seen in Nb/La and Nd isotopes (Fig. 4).The comment maybe is related to the fact that the young Tanna lavas are generally more evolved and there are no basaltic lavas at Yasur.Thus, however, is probably related to the magma ascent which is not discussed in this work.
• Line 185: It's good to mark the absolute age by linear color bar in Fig. 2. Same for the Figs.
4, 5 and 6.We are not sure whether we understand this comment but we have redrawn many of the figures..
• Line 188, Fig. 3: Yellow line is not for visualization in the plot.We changed the colour and think it is better visible now • Line 196, Fig. 4b : What is the yellow zone?The Tanna arc front lavas, a label was added • Line 229: I agree with the "6.5 Ma", but doubt about "2.6".What is the evidence that the pre-rift magmatic activity ended 2.6 Ma ago.We agree that the end of the magmatic activity is not clear and thus deleted this part of the sentence.
• Line 257: The whole section is very interesting.Thank you Review of the manuscript: "Geochemical evidence for slab steepening and rapid mantle wedge replacement during back-arc rifting in the New Hebrides island" By Haase et al.
I have enjoyed reading the manuscript, the authors used details geochemical, geochronological (Ar-Ar), and isotopic (Sr-Nd-Pb) data of basaltic rocks from Futuna trough/island to identify an interesting geochemical compositional change in the lava that formed before and after 2.5 Ma, that gives an indication about the change in mantle source from enriched to depleted ones and related these observations to slab rollback and opening of back-arc basin.After reading this MS, I can say that the authors did a very good job making the MS ready for publication after a few more suggestions.
There are some major comments for the authors to be considered: 1-Can you add a section about the petrographical description of the samples (+ images) in the supplementary section?The petrography is not particularly important for this manuscript and thus we refrained from adding extensive petrography and pictures.2-Ar-Ar dating figure 8 is missing!!!? Added as Figure S3 3-I didn't see any discussion about the Sr isotopes data!! Can you show the relationship between Sr vs. Nd and Pb isotopes?We are cautious about the Sr isotope data as some of the older samples and older data apparently are affected by seawater alteration and were not leached prior to analysis.For example, Sr isotope data for Tanna and Futuna lavas scatter considerably, whereas Nd and Pb isotope data give a more consistent trend.
Below are line (L) comments that hopefully are useful to the authors: L38-41, 49-52: Add reference, please.Added L44-48: Variation in FME can also be temperature-dependent.Gamaleldien et al. 2019 SR (subduction polarity paper) can add to references here.We agree, but the exact origin of the slab component is not the topic of this work and a discussion of the possible formation of the slab component is beyond the purpose of this manuscript.Thus, we do not refer to specific papers on different slab components.L172: You jumped from Fig. 1 in previous sections to Fig. 8, which is missing I can't find it!!!.Sorry, the Ar-Ar figure is now in the supplement.L182: one sentence is needed to summarize this new age data.Added L198: This is not clear, there is no difference between Nd values between old and young Futuna Flank data?And the same for Ce/Pb ratio and even the Tanna samples??This sentence was indeed confusing and we have rewritten it and refer more to figures.We hope our statements are clearer now.L272: Some other ratios Ba/Nb, U/Nb, Pb/Nd, can help in your discussion and show the differences in slab contributions between the two types, please see Gamaleldien et al, 2020, SR (start of subduction paper).I like to see these ratios vs. Pb, Nd, Sr isotopes.These figures can be added to supplementary.We added a figure with Ba/Nb and one with additional incompatible element ratios (new Fig. 4), but U/Nb is quite scattered, probably due to alteration, and we show a figure with Ce/Pb (Fig. 5b) which is very similar to Pb/Nd.Thus, we do not think that more figures help to make our points and the reviewers were satisfied with the number of figures and thought they were convincing.
Fig. 1: Please add a more general map to Fig. 1 to show the location for the general/public readers.The symbol for Tanna Yasur "red triangle" can't be found in the following figures!!!We have redrawn Figure 1 and we hope that locations and symbols can be observed better now.Fig. 2: Can you add the age for Tanna's old and young lava ?? OK, there is not much radiometric age data, but we added the known ages.
2-Ar-Ar dating figure 8 is missing!!!? 3-I didn't see any discussion about the Sr isotopes data!! Can you show the relationship between Sr vs. Nd and Pb isotopes?Below are line (L) comments that hopefully are useful to the authors: L38-41, 49-52: Add reference, please.L44-48: Variation in FME can also be temperature-dependent.Gamaleldien et al. 2019 SR (subduction polarity paper) can add to references here.L172: You jumped from Fig. 1 in previous sections to Fig. 8, which is missing I can't find it!!!.

Fig. 1 :
Fig. 1: Please add a more general map to Fig. 1 to show the location for the general/public readers.The symbol for Tanna Yasur "red triangle" can't be found in the following figures!!! Fig. 2: Can you add the age for Tanna's old and young lava ?? Fig 5: Can you add Ba/La vs Pb isotopic data?Fig 6: Please explain how you did the mixing model?Fig. 7: please label the different components of the figure such as oceanic crust, mantle wedge, …….I think this figure can be more visualized than the present one??
slab may have steepened since 2.5 Ma.Presently, the New Hebrides slab is one of the steepest on Earth and the opening of the Coriolis Troughs indicates that the upper plate stress field became extensional some 2.5 Ma ago.We believe that the most likely explanation is the steepening of the plate which is supported by numerical and experimental models e.g. by Cagnioncle et al (2007, JGR) and Grove et al (2012, Nature).

3.
The balance between slab rollback and slab steepening.The New Hebrides subduction is featured by fast slab rollback.In my opinion, fast slab rollback promotes slab flattening, rather than steepening.Seismic data show that the slab at the New Hebrides is one of the steepest on Earth and it was shown that steep slabs correlate with fast rollback, e.g. by Lallemand et al 2005 and Heuret et al 2005.

Figure 1
Figure 1 was redrawn and we hope it is better now, showing all the locations used in the text.• There are many location names (a bit confusing).The authors know the area very well, but I do have a hard time to follow the names.Please label or draw the boundaries of the geological units as well as the geographic units in the figure explicitly (e.g., Futuna backarc, Futuna flank, Tanna volcanoes, Tana arc front).What is Futuna back-arc (e.g., Line 196)?The Futuna island?Where is Tanna arc front? Figure 1 was redrawn and we tried to avoid too many location names in the text.• Lines 83-102: It's good to discuss the onset of the rifting, although it's debated.Could the authors provide the preferred onset time of the rifting (and the reasons) based on the previous studies?The preferred onset is 3 to 2.5 Ma based on previous tectonic models which is now discussed in the text.• Line 172 Where is Fig. 8? Delete the extra "in".Deleted, the figure is moved to the supplement.

•
Line 284: How to draw this qualified conclusion (e.g., <6% and >60>)?The absolute amounts are of course dependent on the mixing model but the much larger input for the older lavas than for the younger should hold.• Lines 326-328: Why the decreased slab input imply slab steepening?Besides, should the upper plate extension be caused by slab rollback, rather than slab steepening?Numerical models and experimental data suggest that the steeper slabs will effect a relatively narrow portion of the mantle wedge with fluids or melts (see, for example, Cagnioncle et al 2007 JGR and Grove et al 2012 Nature).Thus, the fact that we do not see the slab component in the back-arc after 2.5 Ma suggests to us that the slab steepened.We have explained this in more detail in the text and changed Figure 8. Statistic studies of extension in the upper plate suggest that steep slab roll back and thus cause extensional stress.

Fig 5 :
Fig.1: Please add a more general map to Fig.1to show the location for the general/public readers.The symbol for Tanna Yasur "red triangle" can't be found in the following figures!!!We have redrawn Figure1and we hope that locations and symbols can be observed better now.Fig.2: Can you add the age for Tanna's old and young lava ?? OK, there is not much radiometric age data, but we added the known ages.Fig 5: Can you add Ba/La vs Pb isotopic data?We added Ba/Nb versus 206Pb/204Pb because Ba/Nb shows distinct compositions for the subduction-related lavas and enriched MORB.Fig 6: Please explain how you did the mixing model?The end-member compositions of the